Comfrey is a tall, perennial, herb plant, also known as boneset because it was used to heal broken bones. The word comfrey comes from the Latin word for "grow together". It is a member of the Boraginaceae family, which also include borage, forget-me-nots, lungwort, and brunnera. There are several varieties of comfrey and any can be used to make fertilizer.
Open-pollinated (OP) (Symphytum officinale) varieties are readily available. One that is rapidly gaining in popularity is Bocking 14, a cultivar of Russian comfrey, (S.x uplandicum), developed in the town of Bocking, U.K. Bocking 14 does not produce viable seeds, so you don't have to worry about your plant becoming a nuisance. It also shows good resistance to rust, a fungus disease common to comfreys. However, Symphytum officinale, with its dangling, bell-shaped purple flowers, is great for attracting bees and other pollinating insects.
What Makes It a Good Fertilizer?
With its deep taproot and large root system, comfrey pulls its nutrients from way down in the subsoil, where most other plants can't reach. Comfrey is high in just about every nutrient a plant needs, including the big three, nitrogen, phosphorus and potassium, and many trace elements.
Its high carbon to nitrogen value means that it does not deplete nitrogen from the soil, as it decomposes. It becomes a good source of nitrogen. And it has more potassium than composted manure.
Note: Comfrey leaves are coarse with lots of scratchy hairs that can irritate the skin. It helps to wear gloves when handling them. Long sleeves might not be a bad idea, either.
How to Use Comfrey as Fertilizer
As a liquid, Comfrey leaves decompose into a liquid rather quickly and make a great liquid fertilizer. To make this fertilizer, start with a large container, like a five-gallon bucket. Squish and squeeze as many leaves into as you can into the container. You can weight them down with a rock or brick. Check regularly, but it should take about six weeks for all the leaves to break down into a thick, black liquid. To use on established plants, dilute 1 part comfrey fertilizer in 15 parts water. Use to water and to spray on your plants as a foliar feed. When feeding young plants whose roots could be damaged by strong fertilizer, you may want to dilute it further. Two alternative methods.
- Drill a hole in the bottom of the large container, before adding the leaves. Place a smaller container under the hole in the larger container, to catch the drippings.
- You could add some water to steep the leaves in, but this produces a very strong odor. You are allowing the leaves to rot in the water and the high nitrogen content in the leaves assists in rapid rotting. Since this solution is already somewhat diluted, you will only need to mix this liquid with three parts of water. Used straight, it can be too strong for the roots. Use every time you water. To lessen the odor, use a container with a top. A top and a spigot would be ideal.
- Used as mulch: Add a layer of leaves as a mulch, at the base of plants and in planting beds. You can chop them first, to speed their decomposition, or leave them whole. If the leaves are dry and try to fly away, wet them down and top them off with a layer of compost.
- In compost: You can toss excess comfrey leaves and plants into your compost and still benefit from the released nutrients when you use your finished compost. This is the best use of the stalks, which take longer to breakdown. It even acts as an activator and helps to speed up the composting process.
- In the soil: Line the bottom of planting holes and even containers with a couple of leaves, then plant as you normally would. The leaves will slowly decompose and release their nutrients--with no odor.
- Leaf mold: Because comfrey leaves decompose into a liquid, they cannot be used alone, to make leaf mold. However mixed with other types of shredded leaves they will again act as an accelerator and you will have lovely, earthy, nutrient-rich leaf mold in record time. Leaf mold can be used as a fertilizer, a side dressing or even a potting soil.
Source: Alternative Field Crops Manual
Li, Thomas S. C. “Nutrient Weeds as Soil Amendments for Organically Grown Herbs.” Journal of Herbs, Spices & Medicinal Plants, vol. 4, no. 1, Sept. 1996, pp. 3–8.
Quick Guide to Fertilizing Plants. UMN Extension